LED technology has been promoted as a promising lighting technology to replace energy-inefficient incandescent lamps and mercury-based linear and compact fluorescent lamps. It is often claimed by LED manufacturers that the LED devices have a long lifetime that could be higher than 5 years. However, the electrolytic capacitors used in the power circuit and the electronic controls for LED systems have a limited lifetime, typically 15000 hours (or 1.7 years) at an operating temperature of 105° C. The lifetime of an electrolytic capacitor is highly sensitive to the operating temperature. The lifetime is doubled if the operating temperature is decreased by 10° C. and halved if increased by 10° C. Therefore, the short lifetime of electronic control circuits (sometimes known as ballasts) for LEDs remains one major bottleneck in the utilization of LED technology.
In general, electrolytic capacitors are used in power inverter circuits and electronic control circuits for lighting systems because they provide the necessary large capacitance of the order of hundreds and even thousands of micro-Farads, while other more long-lasting capacitors such as ceramic, polypropylene and metalized plastic film capacitors have relatively less capacitance of several tens of micro-Farads or less. The large capacitance of electrolytic capacitors is usually needed to provide a stable dc link voltage for the ballast circuit to provide stable power (with reduced power variation) for the load; a stable dc power supply in the electronic control for the power inverter circuit.
FIG. 1 shows the schematic of a typical off-line lighting system. An off-line system here means a system that can be powered by the ac mains. The power conversion circuit can adopt a two-stage approach in which an AC-DC power stage with power factor correction is used as the first power stage, which is followed by a second dc-dc power conversion stage for controlling the current for LED load. An alternative to the two-stage approach is to employ a single-stage approach which combines the two power stages into one and such a technique has been reported in many off-line power supply designs. In both approaches, electrolytic capacitors are used to provide the energy storage and buffer so that the difference between the input power and the output power consumed by the load can be stored or delivered by the capacitors.
Regardless of whether a single-stage or a two-stage approach is used, a large capacitance (requiring the use of electrolytic capacitors) is needed as energy-storage to cater for the difference between the input power from the ac mains and the almost constant power of the LED load. The input power of an off-line lighting system is typically a periodically pulsating function as shown in FIG. 1. For example, if power factor is close to one, the input voltage and current are in phase and thus the input power follows a pulsating waveform (similar to a rectified sinusoidal waveform). If the lighting load is of constant power, then the capacitors are needed to absorb or deliver the difference in power between the ac mains and the lighting load as shown in FIG. 1.
An electronic ballast circuit without the use of electrolytic capacitors has been proposed. But the requirement for active power switches in such proposal means that an electronic control board that provides the switching signals for the active power switches is needed and this electronic control board needs a power supply that requires the use of electrolytic capacitors. In general, electrolytic capacitors are needed in a dc power supply for providing the hold-up time (i.e. to keep the dc voltage for a short period of time when the input power source fails.) Power electronic circuits that use active switches usually need a dc power supply for the gate drive circuits that provide switching signals for the active electronic switches. Therefore, it would be useful if a passive electronic ballast circuit can be developed for providing a stable current source for the LED load. A passive ballast circuit without active switches, electronic control board and electrolytic capacitors would be a highly robust and reliable solution that enhances the lifetime of the entire LED system. The remaining challenge is to determine how to provide a stable current source for the LED load based on a totally passive circuit.